How to Repair an Air Conditioner

Need to fix your car's air conditioner? We are a team of ASE certified mechanics that have created this guide for you so you can understand how the system works and what to look for when it doesn't. We also have included popular repair guides with videos which will help you stream line your repairs or see what you are paying for when taking your car in to a repair shop. Let's jump right in!


When looking under the hood at your car's air-conditioner it might seem complicated but it's not. Once you know how it delivers cold air you will see it's really quite simple. Your heater and air conditioner (HVAC) does three jobs; cool the interior of the car, heat the interior up and defrost the windshield. All three modes work together as a complete system. The climate control system is controlled by a main computer which houses the controls for the system and sits in the dash in most cases.

This computer gives commands to the compressor to start pressurizing the refrigerant along with the blend door actuators which direct air flow from the floor, mid and defrost vents. These air door actuators also control air temperature by mixing the hot air from the heater and the cold air from the AC. When one part of the system fails to operate like the air conditioner then one part of the system stops working in this case the cold air.

The AC system has four basic parts: A compressor, which is powered by the engine using a serpentine belt. On hybrid cars the compressor is electrically powered and does the same operation. A condenser which is located in front of the engine radiator and cools the refrigerant from the compressor before it heads to the evaporator located inside the vehicle. This is where the high pressure liquid is released into the evaporator as a low pressure gas and where the coldness is created.  The blower motor then circulates the air throughout the interior.

Before you start doing repairs on the system it's a good idea to see how the a/c system works.

Before You Begin

Start with the car on level ground with the transmission is park and the emergency brake set. The engine should be off, but warm. Wear safety goggles and gloves to protect against injuries. Never loosen or undo any hoses or fitting before discharging the system of refrigerant. The system does contain a slight amount of oil which can be expelled while discharging.

You will see tools and supplies used in the following guides that you will be able to find throughout and at the end of this article, also a list of specific repair manuals. One thing to remember is all air conditioner systems work on the same principle.

Okay, now that you have had a crash course on how the system works let's approach this problem like a mechanic would.

Why Isn't My Air Conditioner Working?

If air is not blowing from the vents it is not the problem of the refrigerant recycling system (A/C) and can be attributed to the blower motor or a vent control actuator. Here is how to tell which; start the engine and turn the air conditioner on. Next, move the fan setting from the highest to the lowest. If you can hear nothing it is a problem with the blower fan.

If you can hear the fan but no air is forced through the vents, or if the air is being generated from the wrong vents this is an actuator problem.

If air is blowing from the correct vents but is not cool (no cold air - blowing warm air) continue down the guide.

When the air conditioner is switched on three things begin to happens, the compressor, blower motor and vent actuators all receive an electrical signal to turn on. This causes the compressor clutch to engage while working the internal parts of the compressor pump. This pumping action compresses the refrigerant so it can circulate around the system.  The following step are presented in order of popularity.

Step 1: Check the A/C System Charge

As your vehicle ages the refrigerant level should be maintained and added too insuring that the system stays full. Because this system is pressurized kind of like a tire it will need to be recharged as it ages. Recharging the system is not to difficult and can be done in about 20 minutes.

If the system is low then you can recharge and monitor how long the charge lasts. If the system lasts up to 6 months then another recharge might outweigh repairing the system which you can do for about $35.00 yourself. A sign of a system being low on charge is that it will produce white vapor from the vents much like your home freezer. This is because the pressure drop inside the evaporator is to excessive causing it to "ice up". Recharging the a/c system will fix this problem.

ac recharge kit

To check the system's "state of charge", connect a gauge or charge kit to the low side hose service port to read the static pressure.

low side ac air conditioner port

This will show the pressure in the system. If the system is flat there is a refrigerant leak that must be discovered and repaired before recharging the system.

no air conditioner system pressure

If the system is full and the static pressure (engine off) is between 70 and 90 psi then continue down the guide of checking the system.

static ac pressure

Step 2: Check the Compressor Operation

In this image the engine is running but the clutch is not engaged. With the system on this can mean one of two things, first the system is low on charge and so the compressor will not turn on via a pressure switch located somewhere on the low pressure side of the system. Or there is an electrical problem of some kind ie: blown fuse, relay and clutch engagement coil or climate control computer shorted. If when you turn the air conditioner on there is a loud screeching noise the compressor has locked up and needs replacement

air conditioner clutch rotation

Before we keep going there is an additional style of compressor that are featured on German cars (BMW, Mercedes Benz and Volkswagen) which does not have a clutch. One of these compressors is featured on the image below in which there is no clutch and no wires to an activation coil.

As you can see the portion of the pulley hub where the clutch would be located is directly bolted to the input shaft of the compressor. These compressors have an internal valve which is located at the rear of the compressor to begin the process when activated. For these cars there will be one variable in testing which we will get to further down in the guide.

no clutch air conditioner compressor

On clutch style compressors with the system still on and set to the coldest setting and highest fan speed locate the compressor. Using a flashlight and inspect the clutch operation it will be doing one of two things; first, it will be cycling on and off which means the system is low on charge or the condenser fan is not operating or there is blockage in the expansion or orifice tube.

If the clutch is doing nothing then its time to connect a gauge or charge kit to see if the system is completely flat or is just so low on charge. When the system is low or overcharged a pressure switch tells the compressor when to shut down to avoid damage. On internal valve compressors i.e. German cars you must use a test light to see if the signal wire is getting power or not. This style of system does not cycle when it's low it just shuts down so its best to check the systems charge.

Step 3: Check the Condenser Fan

Start the engine and set the air conditioner to its coldest setting. The first thing to look for is the condenser fan being on (if equipped). This fan should turn on within a minute of the AC system being switched on. If this fan is not on then this is a problem that must be checked out. If the vehicle is rear wheel drive and has a mechanical fan look to see if the fan is "freewheeling" which is an indication of the fan clutch being bad and needing replacement. The condenser must have air being pulled or pushed through it for the system to work.

Step 4: Check the Climate Control Electrical System

Observe the climate control panel to see if there are any lights that are blinking which indicates there is some kind of failure in which a code must be read. Each method of retrieving these codes is different to each manufacturer. These codes will guide you to the circuit or system that is having a problem. If everything looks okay on the panel or the panel lights are out completely please continue down the guide.

air conditioner panel

Locate the car's fuse panel under the dash or power distribution center under the hood. Using a test light check all related fuses such as the BCM, climate control, heater, blower motor and for the air conditioner. You can use your car's owner's manual to help locate the fuse.

If the new fuse blows once it's been replaced there is a short in the system. Most of the time this short can be related to the compressor clutch coil in which case it needs to be replaced.

test fuse

If all related fuses are okay the next part in the system that has a high failure rate is the a/c relay. This relay supplies main power to the compressor via the system fuse. Locate the relay in the fuse panel or power distribution center using the owners manual.

Once located check the terminals of the relay for power and ground and then replace the relay to recheck the system. You can swap this relay for a similar one in the car such as the power windows or door locks to test it, many of these relays are the same.

inspecting relay terminals

A pressure sensor is used to measure the amount of refrigerant is in the system. When this sensor goes bad the compressor will not turn on. To test this sensor remove the wiring connector and insert a jumper wire between the two terminals of the connector. If the compressor turns on the sensor is bad or the system is low or overcharged. If nothing happens use a test light to see if there is 12 volts at one of the wires. If no power is present suspect the climate control computer being bad.

air conditioner pressure sensor

If you have perform all of this tests and the system is still not working a pin to pin voltage and continuity check is in order. This test sounds harder than it is, using a wiring diagram for you car you basically you are required to test each wire for power or ground and then continuity to make sure each wire is fully connected.

If the engine is overheating or low on coolant the air conditioner will be weak or not work at all. If you have been driving for sometime and you noticed the system stops working the first thing to check it the engine temperature gauge or warning light.

Watch the Video!


Tree leaves, plastic bags and dirt can get logged in the condenser hindering the systems performance. Remove any obstructions such as plastic bags from the condenser area. Also use a garden hose to clean the condenser with a high pressure nozzle from time to time. This works really well and helps the air conditioner work more efficiently. The compressor is the main mechanical part of the system which is driven by the engine's serpentine belt. This belt should be in good working order and replaced if worn.

Some cars are equipped with the cabin air filter which is much like an engine air filter that can plug causing restricted air flow. This filter should be replaced when air flow becomes restricted.

If repairs have been made, the system will need to be vacuumed down and recharged to remove damage causing moisture. If you don't vacuum the system the A/C will not be as cold because of the air that is trapped inside the system once it has been opened.

If you have any questions about car air conditioners please visit our forum. If you need car repair advice please ask our community of mechanics are are happy to help.

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